Signal handling

[bartvm]

The current platoon-launcher script seems a bit messy with handling its processes. I added an option that allows the user to gracefully signals in the controller. So if you add this to the controller:

class CustomController(Controller):
    def __init__(self, ...):
        self._stop = False
        signal.signal(signal.SIGTERM, self.stop)
        signal.signal(signal.SIGINT, self.stop)

    def stop(self, signum, frame):
        self._stop = True
        signal.signal(signal.SIGTERM, signal.SIG_DFL)
        signal.signal(signal.SIGINT, signal.SIG_DFL)

    def handle_control(self, req, worker_id):
        if self._stop:
            return 'stop'

your workers will just get a stop command the next time they make a request. This way things can terminate gracefully on clusters or when the user presses CTRL-C to end an experiment.